1. Field of the Invention
The present invention relates to a flow control apparatus for supplying fluid oil discharged from an oil pump to a hydraulic operating apparatus like power steering device through a metering orifice and returning excess fluid to the oil pump through a bypass path.
2. Description of the Related Art
In a prior art of a flow control apparatus, there has been known Japanese laid-open patent publication No. 2002-168358. This publication discloses a flow control apparatus for supplying operating fluid discharged from an oil pump to a hydraulic operating device through a metering orifice and returning excess fluid to the oil pump according to fluid pressure difference between upstream and downstream of the metering orifice. The disclosed flow control apparatus comprises a hollow rod formed on a spool and a metering orifice formed between an opening of a union and the rod passing through the opening of the union to introduce operating fluid pressure of downstream of the metering orifice to a pressure room located at opposite end of the spool through a communicating path in the rod. According to this prior art, a communicating path in a flow control valve housing is not required because fluid pressure of downstream of the metering orifice is introduced to a pressure room located at opposite end of the spool through a communicating path in the rod. Therefore, the flow control valve housing can be become small in size, and drilling process of the flow control valve housing and plugging operation, for example, press fitting a ball to the drilling hole for fluid sealing can be eliminated. As a result, assembling efficiency of the flow control apparatus can be improved, and reliability can be improved also due to no operating fluid leakage to outside of the flow control apparatus.
Such a flow control device requires a relief pressure mechanism to protect the oil pump from overload by reducing fluid pressure in the room at opposite end of the spool and retracting of the spool when fluid pressure in a hydraulic operating apparatus reaches a predetermined threshold pressure.
However, in the prior art described above, the relief pressure mechanism described above requires complex structure, for example, a ring shape seat valve and so on, because the rod providing a communicating path of fluid pressure between downstream of the metering orifice and the room at opposite end of the spool is arranged in an area from downstream of the metering orifice to the room at opposite end of the spool along center line of the spool. Further, a relief valve sub-assembled in advance can not be used.
Therefore, assembling process of flow control apparatus becomes complex, and it results increasing in cost because relief pressure setting is necessary to be performed after assembling of the flow control apparatus.